This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-336395 filed on Nov. 2, 2000.
The present invention relates to a vehicle alternator control device and method for controlling the electric power generating operation of an alternator for vehicles.
In an alternating current generator (alternator), a small alternating current (a.c.) voltage is generated in each armature coil even when a rotor is turned without being supplied with a field current to the field coil. This is because residual flux remains in the field poles of the rotor.
In JP-U-62-44698, a control circuit is provided to detect if a rotor of an alternator starts to turn, that is, if a vehicle engine is started, by detecting the frequency of the induced voltage of the armature coil as a result of residual magnetization in the field poles of the rotor. The control circuit starts supplying a field current to the field coil when the engine is detected to have started. By thus detecting that the engine has started based on the voltages induced in the armature coil, the signal wire from the vehicle used to indicate the on/off state of an ignition switch can be eliminated, and the wiring can therefore be simplified.
However, there is a significant difference between when the engine is starting (cranking) and when the engine is stopping in the residual magnetization of the field poles and behavior when the engine is turning. Therefore, the engine rotation cannot be detected with sufficient precision if the same frequency is used to detect when rotation starts and when rotation stops.
In general, the voltage signal appearing in the armature coil when the engine starts is extremely weak because it results only from the residual flux in the field poles. If the flux linkage to the armature coil is constant, the amplitude and frequency of the voltage induced in the armature coil are proportional to the speed of the rotor. This frequency can be detected by digitizing the voltage signal in the armature coil using a voltage comparator or other means and then applying a specific digital operation.
To detect the rotor speed using the weak signal resulting from the residual flux, the reference voltage used for comparison by the voltage comparator must be set to a low voltage. If this reference voltage is too low, detection errors will arise due to noise or other external signals. It is therefore necessary to set the reference voltage of the voltage comparator high enough so that the comparison will not be influenced with the external noises. However, if the reference voltage of the voltage comparator is too high, a digital pulse cannot be generated until the induced voltage of the armature coil becomes high enough, and the detectable rotor speed thus rises.
After the engine is started with a starter and the engine is firing normally, it reaches an idling speed after the engine speed rises to a specific level. The engine starting can be detected even if the frequency used to detect the speed is set relatively high. Particularly in winter when the ambient temperature is low, the engine is controlled to start at a higher initial idling speed than in warm weather in order to warm up the engine quickly.
However, if the engine start detection speed is set higher than a speed equivalent to the normal idling speed setting, it may not be possible to sustain power generation when the engine is idling, such as when waiting at a traffic signal.
It is proposed to avoid this problem that the number of turns of the armature coil can be increased in order to lower the speed at which current generation starts. However, this results in a drop in the output current at high engine speeds and hence is not a good solution because the balance between charging and discharging a vehicle battery is impaired.
JP-A-6-292329 proposes to change coils according to the speed. However, this is not a practical solution, because the circuit scale then becomes large.
U.S. Pat. No. 5,429,687 proposes to improve magnetic characteristics by heat treating the field poles and using magnetic annealing causing a transition to a crystalline phase that increases the residual flux. However, heat treatment of the field poles with high heat capacity requires a large scale heat treatment system.
The present invention addresses these problems, and has an object to provide a vehicle alternator control device that can improve precision of engine speed detection without increasing circuit scale or impairing battery charge-discharge balance.
According to the present invention, a vehicle alternator control device starts an operating voltage supply operation when the frequency of an a.c. output voltage of an alternator is greater than a first reference frequency, and stops the operating voltage supply when the frequency of the a.c. output voltage drops below a second reference frequency lower than the first reference frequency. Detection errors due to noise can be prevented and detection precision can be improved because engine start detection uses a high frequency corresponding to the first reference frequency.